Optical communication networks require an optical transmitter in each optical network unit. To eliminate the need for the transmitters, an optical LAN in star topology with a central laser has been constructed, as described in Electronics Letters, Sep. 1, 1994, Vol. 30, No. 18, pages 1506-1507. The central laser provides reference pulses which are transmitted to all network units. In each network unit, the received reference pulses are modulated with the respective digital data stream via an electro/optical switch using the TDMA technique and subsequently reflected into the communication network. Because of the differences in propagation time which are caused, for example, by temperature variations, problems relating to synchronization can arise. The reflection of the modulated reference pulses is lossy and may lead to interferences with the unmodulated reference pulses. In addition, the optical network units have to be synchronized with respect to each other.